BACKGROUND:Bullet Bills are one of the most common enemies in the Koopa Troop's arsenal. They are large missiles with eyes, and often have mouths and/or
arms as well. They are fired out of cannons known as Blasters, which are usually invincible, but can be destroyed in some games.
Bullet Bills are dangerous, but predictable. They are essentially projectiles, without any intelligence, even though they display some animal-like characteristics. There are some more intelligent Bullet Bills which can follow their intended target to a degree, but most are relatively unintelligent.
While Bullet Bills and their cannons are manufactured by the Koopa Troop, Booster, who loves tinkering with machines, has managed to construct some of his own.

EVOLUTION:Bullet Bills first appeared in Super Mario Bros, and have become one of the series' most commmon enemies since then. Originally, they were bullets with eyes and arms, an appearance which remained the basic design for quite a while,
although in some cases they were drawn with mouths.
Finally, in Super Mario World 2, they gained the sharp-toothed smiley face that drawings often showed them with. All of these designs were
carried on and combined in Super Mario RPG, where they had both arms and the smiley face.
Some might begin to think that Bullet Bills were starting to change over time. The problem is, Super Mario World 2 takes place long before any of the other games, chronologically - and at any rate, Bullet Bills can't really "evolve" as they are mechanical.
Super Mario 64 and Yoshi's Story continued on with Bullet Bills that have mouths, but no arms. Then in Paper Mario they returned to the "arms no mouths" style. Experts now believe that there are simply multiple designs for Bullet Bills
which may be used interchangeably.
Super Mario Sunshine saw the most radical redesign of Bullet Bills in some time. They lost the traditional "angry eyes" for bulgy googly eyes and their nose-cones became more detailed. They also have small "fins" near the back.